Microelectronic devices are generally fabricated on semiconductor substrates as integrated circuits. A complementary metal-oxide-semiconductor (CMOS) field effect transistor is one of the core elements of the integrated circuits. Dimensions and operating voltages of CMOS transistors are continuously reduced, or scaled down, to obtain ever-higher performance and packaging density of the integrated circuits.
One of the problems due to the scaling down of CMOS transistors is that the power consumption keeps increasing. This is partly because leakage currents are increasing (e.g. due to short channel effects) and because it becomes difficult to decrease the supply voltage. The latter is mainly due to the fact that the subthreshold slope is limited to minimally about 60 mV/decade, such that switching the transistor from ON to OFF needs a certain voltage variation and therefore a minimum supply voltage.
Tunnel field-effect transistors (TFETs) are typically advertised as successors of metal-oxide semiconductor field-effect transistors (MOSFETs), because of their absence of short-channel effects and because of their resulting low off-currents. Another advantage of TFETs is that the subthreshold slope can be less than 60 mV/dec, the physical limit of conventional MOSFETs, such that potentially lower supply voltages can be used. However, TFETs typically suffer from low on-currents, a drawback related to the large resistance of the tunnel barrier.
To increase the on-current of a silicon TFET, suggestions have been made in literature by Bhuwalka et al. (IEEE transactions on electron devices Vol. 52, No 7, July 2005) to add a small (about 3 nm wide) section of highly-doped Si(1−x)Gex at the tunnel barrier. The Si(1−x)Gex has a smaller band gap than Si such that the effective tunnel barrier width decreases due to the presence of this section. However, these structures with the Si(1−x)Gex section can still not compete with conventional MOSFETs because of their low on-currents.
Verhulst et al. [A. S. Verhulst et al, J. Appl. Phys., 104(6):064514, 2008] have shown that the ON current of a tunnel field-effect transistor can be boosted by using a p-doped Ge (or p-doped SiGe) source in combination with a Si channel and n doped drain. However, the increase in ON current depends strongly on the abruptness of the (p) doping profile at the (Si)Ge/intrinsic Si tunneling junction (see FIG. 1). This is a challenging task to achieve, as the p-type dopants (e.g. B dopants) readily diffuse into the Si channel due to the thermal budget during the device fabrication.